Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to heart tissue. If sufficiently severe, cardiac ischemia results in an acute myocardial infarction (AMI), also referred to as a heart attack. With AMI, a substantial portion of heart muscle ceases to function because it no longer receives oxygen, usually due to significant blockage of the coronary artery. Generally, AMI occurs when plaque (such as fat, cholesterol, and calcium) builds up and then ruptures in the coronary artery, allowing a blood clot or thrombus to form. Eventually, the blood clot completely blocks the coronary artery and so heart tissue beyond the blockage no longer receives oxygen and the tissue dies. In many cases, an AMI proves fatal because too much tissue is damaged to allow continued functioning of the heart muscle. Indeed, AMI is a leading cause of death here in the United States and worldwide. In other cases, although the AMI itself is not fatal, it strikes while the victim is engaged in potentially dangerous activities, such as driving vehicles or flying airplanes, and the severe pain and possible loss of consciousness associated with AMI results in fatal accidents. Even if the victim survives the AMI, quality of life may thereafter be severely restricted.
Often AMI is preceded by episodes of cardiac ischemia that are not sufficiently serious to cause actual permanent injury to the heart tissue. Nevertheless, these episodes are often precursors to AMI. Episodes of cardiac ischemia may also trigger certain types of arrhythmias that may prove fatal, particularly ventricular fibrillation (VF) wherein the ventricles of the heart beat chaotically, resulting in little or no net flow of blood from the heart to the brain and other organs. Indeed, serious episodes of cardiac ischemia (referred to herein as acute myocardial ischemia) typically result in either a subsequent AMI or VF, often within twenty-four four hours, sometimes within only a half an hour or less. Accordingly, it would be highly desirable to provide a technique for reliably detecting episodes of acute myocardial ischemia so that the victim may be warned and medical attention sought. If properly warned, surgical procedures may be implemented to locate and remove the growing arterial blockage or anti-thrombolytic medications may be administered. At the very least, advanced warning would allow the victim to cease activities that might result in a fatal accident. Moreover, in many cases, AMI or VF is triggered by strenuous physical activities and so advanced warning would allow the victim to cease such activities, possibly preventing AMI or VF from occurring.
Many patients at risk of cardiac ischemia have pacemakers, ICDs or other medical devices implanted therein. Accordingly, techniques have been developed for detecting cardiac ischemia using implanted medical devices. In particular, techniques have been developed for analyzing intracardiac electrogram (IEGM) signals in an effort to detect cardiac ischemia. See, as examples, the following U.S. Pat. Nos. 5,113,869 to Nappholz; 5,135,004 to Adams et al.; 5,199,428 to Obel et al.; 5,203,326 to Collins; 5,313,953 to Yomtov et al; 6,501,983 to Natarajan, et al.; 6,016,443, 6,233,486, 6,256,538, and 6,264,606 to Ekwall; 6,021,350 to Mathson; 6,112,116 and 6,272,379 to Fischell et al; 6,128,526, 6,115,628 and 6,381,493 to Stadler et al; and 6,108,577 to Benser. Many ischemia detection techniques seek to detect ischemia by identifying changes in the elevation of the ST segment of the IEGM that occur during cardiac ischemia. The ST segment represents the portion of the cardiac signal between ventricular depolarization (also referred to as an R-wave or QRS complex) and ventricular repolarization (also referred to as a T-wave). The QRS complex usually follows an atrial depolarization (also referred to as a P-wave.) Strictly speaking, P-waves, R-waves and T-waves are features of a surface electrocardiogram (EKG). For convenience and generality, herein the terms R-wave, T-wave and P-wave are used to refer to the corresponding internal signal component as well.
Alternative techniques for detecting cardiac ischemia have also been developed that do not necessarily rely on ST segment elevation. One such technique is set forth in U.S. patent application Ser. No. 10/603,429, entitled “System And Method For Detecting Cardiac Ischemia Using An Implantable Medical Device,” of Wang et al., filed Jun. 24, 2003, which is incorporated by reference herein. Rather than examine the ST segment, the technique of Wang et al. instead examines post-T-wave segments, i.e. that portion of the cardiac signal immediately following the T-wave. In one example, the onset of cardiac ischemia is identified by detecting a sharp falling edge within post-T-wave signals. Another technique for detecting cardiac ischemia based on T-waves is set forth in U.S. patent application Ser. No. 10/603,398, entitled “System And Method For Detecting Cardiac Ischemia Based On T-Waves Using An Implantable Medical Device,” of Min et al., filed Jun. 24, 2003. With the technique of Min et al., cardiac ischemia is detected based either on the total energy of the T-wave or on the maximum slope of the T-wave. See, also, U.S. patent application Ser. No. 11/043,612, of Kil et al., filed Jan. 25, 2005, entitled “System and Method for Distinguishing Among Ischemia, Hypoglycemia and Hyperglycemia Using an Implantable Medical Device.” In addition, see U.S. patent application Ser. No. 11/394,724, of Ke et al. entitled “System and Method for Detecting Cardiac Ischemia in Real-Time using a Pattern Classifier Implemented within an Implantable Medical Device”.
Thus, a variety of techniques have been developed for detecting cardiac ischemia based on changes in IEGM morphology. However, it can sometimes be difficult to distinguish variations in IEGM morphology caused by cardiac ischemia from variations due to changes in patient posture. In this regard, various gravity-dependent and position-based hemodynamic and pulmonary affects on heart rate, blood pressure and oxygen consumption have been documented. See, Jones et al., “Body Position Change and Its Effect on Hemodynamic and Metabolic Status,” Heart Lung. 2004 September-October; 33(5):281-90. Jones et al. report that heart rate, blood pressure and oxygen consumption are highest in the sitting position compared to the lying (supine) positions and lowest in the left side lying position. Changes in these parameters can potentially affect IEGM morphology. Also, it has been reported that patient posture can affect QT variability (where QT variability refers to a beat-to-beat variability in the duration of the interval between the Q-point of a QRS complex and the subsequent T-wave.) See, Yeragani et al., “Effect Of Posture and Isoproterenol on Beat-To-Beat Heart Rate and QT Variability,” Neuropsychobiology. 2000; 41(3):113-23. In particular, Yeragani et al. found that the QT variability index is significantly higher in the standing that in the supine posture.
Moreover, the present inventor and her colleagues have observed that an atrial paced depolarization integral (PDI) varies due to patient posture. Atrial PDI is a well-known parameter derived from an integral of certain morphological features of the IEGM derived from a paced atrial beat. (For a description of PDI, also sometimes referred to as a depolarization gradient, see U.S. Pat. No. 4,759,366, to Callaghan.) In one example, atrial PDI was derived for ten patients under three different body positions: supine, right-side lying and left-side lying. It was observed that when the patient was first in the supine position and then switched to the right side lying position, the mean of the atrial PDI decreased by 5.6±8.9%. The standard deviation of the atrial PDI decreased by 36.8±7.3%. Similarly, when the patient was first in the supine position, and then switched to the left side lying position, the mean of the atrial PDI decreased by 1.6±3.5%, and the standard deviation decreased by 52.2±29%. In other words, baseline values of atrial PDI can vary depending upon patient posture. In addition to changes in baseline values, transient changes also occur while the patient changes from one posture or body position to another.
Hence, changes in patient posture can affect metrics derived from IEGM morphology and can thereby potentially affect the reliability of IEGM-based cardiac ischemia detection. Accordingly, it would be desirable to provide techniques for improving the reliability and specificity of ischemia detection by accounting for changes in posture and it is to this end that aspects of the present invention are directed. Posture changes can also potentially affect the detection of other medical conditions besides ischemia and so aspects of the invention are also directed to that end as well. It would also be desirable to provide new and improved techniques for detecting the actual changes in posture and still other aspects of the invention are directed to that end.